1. Field
The present invention generally relates to an image forming apparatus such as a copier, facsimile, and printer, capable of improving color stability of an image, and more specifically, to an image forming apparatus capable of improving color stability of an image by correcting image forming conditions of the image forming apparatus based on information obtained by measuring colors of a toner image output from the image forming apparatus, a method of controlling the image forming apparatus, and a recording medium storing an image forming control program.
2. Background
In image forming apparatuses that form a toner image using electrophotographic method, if an amount of charge on toner in a developer in a developing device is not stable, the developing density may fluctuate, thus causing colors in the toner image to be unstable. Since the amount of toner in the developer decreases as toner is used for the developing process, toner is constantly supplied to the developer to keep a toner density in the developer to be within a predetermined range. While the amount of charge on the toner in the developer gradually increases as the toner is agitated with carrier particles in the developer, the amount of charge on the toner, which is expressed as a charge-to-mass ratio “Q/M”, may not be sufficiently high enough especially when images requiring a large amount of toner are successively printed. This causes more toner particles to be adhered to the latent image, thus increasing the developing density. On the other hand, in case of sequentially printing images requiring less toner, the amount of charge on the toner, which is expressed as Q/M, increases such that the developing density decreases.
Japanese Patent Application Publication No. 2001-343827 describes an image forming apparatus, which forms a test toner image on a latent image carrier and detects a toner adhesion amount per unit area of the test toner image. The amount of toner to be supplied to the developer is determined based on the detected toner adhesion amount, thus keeping the charge amount of toner to be within the predetermined range. With this toner adhesion amount stabilization process, the fluctuations in developing density are suppressed such that colors of the toner image are stabilized.
The above-described toner adhesion stabilization process has drawbacks such that it requires printing of test toner images in addition to printing of images (“user images”) requested by a user, thus increasing the overall printing costs and lowering productivity in printing the user images. Further, the user is required to sort the test toner images from the user images after being printed.
Japanese Patent Application Publication No. 2010-271595 discloses an image forming apparatus that measures colors of the user image formed on the recording sheet, and corrects image forming conditions to stabilize the toner adhesion amount based on the measured colors such that printing of the test toner image is not necessary. However, since the colors obtained from the user image are multi-colors, controlling the image forming conditions of each one of primary color images based on the measured multi-colors has been difficult. More specifically, colors used by the image forming apparatuses are mainly classified into primary colors and multi-colors. The primary colors are reproduced using only one type of toner. If there are four types of toner including yellow toner, magenta toner, cyan toner, and black toner, any one of the colors that can be reproduced using one type of toner is referred to as the primary color. The multi-colors are reproduced using more than one type of toner, such as by superimposing toner images of different primary colors one above the other. Since the developing density of each of the primary colors that constitute the multi-colors in the user image cannot be obtained directly from the measured multi-colors in the user image, correcting the image forming conditions of the primary color images based on the measured multi-colors in the user image has been difficult.